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Abstract

Background: Small pneumothoraxes (PTXs) may not impart an immediate threat to trauma patients after chest
injuries. However, the amount of pleural air may increase and become a concern for patients who require positive
pressure ventilation or air ambulance transport. Lung ultrasonography (US) is a reliable tool in finding intrapleural
air, but the performance characteristics regarding the detection of small PTXs need to be defined. The study aimed
to define the volume threshold of intrapleural air when PTXs are accurately diagnosed with US and compare this
volume with that for chest x-ray (CXR).

Methods: Air was insufflated into a unilateral pleural catheter in seven incremental steps (10, 25, 50, 100, 200,
350 and 500 mL) in 20 intubated porcine models, followed by a diagnostic evaluation with US and a supine
anteroposterior CXR. The sonographers continued the US scanning until the PTXs could be ruled in, based on the
pathognomonic US “lung point” sign. The corresponding threshold volume was noted. A senior radiologist
interpreted the CXR images.

Results: The mean threshold volume to confirm the diagnosis of PTX using US was 18 mL (standard
deviation of 13 mL). Sixty-five percent of the PTXs were already diagnosed at 10 mL of intrapleural air; 25%,
at 25 mL; and the last 10%, at 50 mL. At an air volume of 50 mL, the radiologist only identified four out of
20 PTXs in the CXR pictures; i.e., a sensitivity of 20% (95% CI: 7%, 44%). The sensitivity of CXR increased as a
function of volume but leveled off at 67%, leaving one-third (1/3) of the PTXs unidentified after 500 mL of
insufflated air.

Conclusion: Lung US is very accurate in diagnosing even small amounts of intrapleural air and should be
performed by clinicians treating chest trauma patients when PTX is among the differential diagnoses.